Printed circuit boards (PCBs) are typically constructed from two or more layers sandwiched together but separated by dielectric material. Layers can have different thicknesses and different dielectric material can be used within a PCB. Routing or other kinds of copper structures can be implemented at all layers. The outermost layers (top and bottom) of the PCB can have components mounted on their outside surfaces. Multilayer PCBs provide an important advantage over single layer structures in that a multilayer PCB has more routing space in a smaller footprint, which is useful for today's design imperative of smaller-size components.
Vias interconnect traces on different PCB layers and connect layers to power and/or ground planes. The physical properties of a via are dictated by board geometry and available space, and also by application. For example, in high-speed signaling applications, impedance-matched transitions are required between layers, particularly as frequencies in the Gigahertz range are utilized on the PCB.
In high-speed signaling designs, impedance continuity is essential for all of the interconnect elements including traces, connectors, cables and the like. Among all of these interconnect elements, the via presents the greatest obstacle to achieving impedance control, because traditional PCB process flow does not allow fabrication of coxial vias. Coaxial via technology is greatly needed for high-frequency applications as it would enable true signal impedance continuity, provide an excellent return path for ground (GND), and efficiently reduce via-to-via crosstalk and via-to-trace crosstalk.
One prior approach has been to surround a signal via with multiple GND vias. With this approach, the return path and impedance control are greatly improved. However, the additional GND vias consume valuable space on the footprint of a multilayer PCB. To save space, another prior approach was to split the via into four pieces so that one pair serves as signal vias and the other pair serve as GND vias. Unfortunately, performance of this dissected-via approach is unacceptable. Thus, there remains a heartfelt need for improved impedance control and performance in vias configured in multilayer PCBs.